The present invention relates to a gear mechanism of the speed-increasing or speed-reducing type, intended for example to drive, or be driven by, turbine engines.
These mechanisms, which can also be viewed as fixed-ratio transmissions, operate at high speed and are responsible for power losses that manufacturers seek to diminish by working with the various known factors that cause them. Without particular treatment, these losses represent approximately 1% of the total power, and are hence far from negligible for high-power engines.
In the course of research into this problem, the manufacturers have observed that 90% of the losses are generated in the total absence of load, i.e. when the engine is idling. They are due mainly to control of the amount of lubricant and coolant injected between the gear teeth, to gas turbulence created when the toothed parts operate at high speed, and to friction in the bearings supporting the rotating shafts to which said toothed parts are attached.
The traditional design of these mechanisms is as follows: the toothed parts are accommodated in closed casings to which the bearings supporting the rotating shafts of said toothed parts are joined. The casings thus delimit a closed, protected space, in which the lubricating/cooling oil injected into the gears is recovered. The oil is then collected in an external storage tank before being reinjected after cooling into the meshing zone of the teeth of the toothed parts.
One of the most effective ways to reduce losses, particularly that described in U.S. Pat. No. 3,489,034, consists of creating a partial vacuum inside the casing. In this case, the losses due to turbulence caused by the high peripheral speeds of the teeth are considerably reduced. Creation of a partial vacuum thus eliminates about half the total loss, and this can be decreased still further by altering the flowrate of the lubricating/cooling fluid past the bearings, etc.
As an alternative to creating a vacuum, and to obtain a similar result, it has also been proposed that a low-density gas of the helium type be injected into the casing, replacing the air and thus reducing the density of the gas phase in the casing.
The pressure reduction obviously affects the overall design of the mechanism. Thus, the lubricating/cooling fluid recirculation system is obviously affected by the pressure value inside it. The substantial drop in gas pressure demands consideration of the behavior of the engine if the system creating the pressure should break down, and the repercussions of a possible malfunction in the recirculation system.
These problems are what the mechanism on which the present invention is based sets out to resolve. In particular, the existence of a low gas pressure requires mechanical means to be established to assist in the transfer of lubricating/cooling fluid (usually oil) from the casing to the oil storage tank. In fact, the oil is usually transferred by means of a pump, the operation of which can however be impeded by cavitation phenomena because of the pressure inside the casing.
The chief goal of the present invention is to propose a configuration avoiding all cavitation problems when oil is pumped from the casing into the oil reservoir. Very generally, the invention consists of a control device to create a rarefied atmosphere in a gear mechanism.
According to a second related goal, the mechanism according to the invention must continue to operate, at least temporarily, even if the system creating the low pressure and/or the evacuation means fails.
These goals, and others that will emerge from reading the detailed description of the invention, are achieved by the gear mechanism according to the invention which is more specifically composed of at least two toothed parts accommodated in a casing, each part being attached to a rotating shaft in bearings joined to the casing; of a storage tank for lubricating/cooling oil to be injected into each meshing zone of the toothed parts; of means for conveying the oil from said tank to the meshing zones in order to inject it there at a pressure greater than the pressure prevailing in the casing; of means for reducing the pressure in the casing; and of means for transferring the oil to the storage tank.